Sankarmagesh Rajan is correct. But, if you have Tableau Server core licenses, then that comes with guest access which would allow you to embed the visualization in a website, which could then be viewed by anyone without logging in. And, if your data is not confidential, does not include PII, etc. you could use Tableau Public, which is essentially a freely available version of Tableau Server.

It's a really significant (and expensive) change. It's something you're going to really need to evaluate the value of before diving into it. And it has a lot of impact on the architecture of your Tableau Server environment.

What is the use case for sharing your dashboards publicly? There may be another solution.

Without a core license, you can't do that. You can look into core licensing, but as I said, it's pretty expensive, so it may not be a viable approach. The only other option that I can think of is using Tableau Public, but that's publicly available to the world--if your data is such that that's not an issue, then go for it. Lots of people use Tableau Public in this way. For more see Tableau Public Frequently Asked Questions

The only solution I can think of is to invest in core licensing and make the workbooks publicly accessible. To share them, you'd have to embed them on a website. In that case, they would technically be open to the world, so you'd need to implement security on your website such that only those 50 people can access it.

Of course, the other option is to purchase 50 user licenses, which I believe will be the least expensive option in the long run.